1. Field of the Invention
The present invention relates to a fuel-injection system for an internal-combustion engine.
2. Description of Related Art
Known, in the framework of compression-ignition engines for motor vehicles, are injection systems (the so-called common-rail systems) consisting of a plurality of electro-injectors supplied by a common storage volume of fuel under pressure.
In particular, operation of said injection systems envisages that a low-pressure priming pump will draw the fuel from a tank and will make it available to a high-pressure pump. The high-pressure pump compresses the fuel up to the injection pressure and makes it available to a common storage volume, which supplies the electro-injectors.
One of the functions of the common storage volume is that of dampening the pressure oscillations caused by the delivery of fuel from the high-pressure pump to the storage volume and by the extraction of fuel caused by opening of the electro-injectors.
In detail, the electro-injectors are supplied by the common storage volume and inject the fuel nebulized at high pressure into each of the combustion chambers of the respective engine cylinders.
As is known, to obtain a good nebulization of the fuel, this must be brought to a very high pressure, for example, in the region of 1600 bar in the conditions of maximum load. The need to meet recent standards on the limits of pollutants present in the exhaust gases of engines that are to be installed on automobiles imposes that the conditions of operation of the electro-injectors should be accurately controlled, and in particular that the pressure of fuel supply into the injection system should be reproducible as accurately as possible with respect to what is mapped in the electronic control unit. The conditions of low/medium load, which carry the most weight in the final evaluation of the content of pollutant substances in the exhaust gases of the engine, are the most critical. It is possible to limit the pressure oscillations in the common storage volume within acceptable values if the accumulation volume is of approximately two orders of magnitude greater than the amount of fuel taken in by each electro-injector in each combustion cycle. This common storage volume is generally very cumbersome and, hence, of critical importance as regards its installation on the engine.
For controlling the pressure in the common storage volume following the indications supplied by the control unit, there have been proposed injection systems comprising a solenoid valve for regulation of the pressure, which is set on the pipe that sets the pump in communication with the common storage volume.
The pressure-regulation solenoid valve discharges into the tank the fuel pumped in excess with respect to the fuel taken in by the electro-injectors.
Following upon opening of the electro-injectors there occur pressure drops between the electro-injectors themselves and the common storage volume. Said pressure drops are dampened by the storage volume in a way that is the more effective the greater the volume of the storage volume itself.
The aforesaid dampening, however, envisages a transient, following upon opening of the electro-injectors, during which, in the pipe that connects the electro-injector to the common storage volume, the pressure undergoes marked variations.
If, in use, opening of an electro-injector is required during said transient (in the case where, for example, two successive injections that are particularly close to one another are required), it may happen that, on account of the variations in pressure that are not yet exhausted, there is no correspondence between the effective injection pressure and the desired one, with consequent sub-optimal operation of the electro-injectors and, hence, of the internal-combustion engine itself, with an increase in emission of pollutant substances.
The duration of the transient is moreover adversely affected by the fact that the fuel is in general practically <<stationary>> inside the common storage volume in the periods of absence of injection.